1. Field of the Invention
The present invention relates to a laser processing apparatus capable of increasing a focused beam diameter.
2. Description of the Related Art
Generally, a laser processing apparatus which performs processing on a workpiece by using a laser beam is equipped with a light-focusing optical section for focusing or condensing the laser beam supplied from a laser source and entering a processing head, and for emitting the focused or condensed beam from the processing head. The light-focusing optical section includes a desired number of lenses, and provides a predetermined collecting angle (also referred to as a convergent angle, in the present application) to the entered or input laser beam. The laser beam provided with the convergent angle forms a light focusing point or focal spot, corresponding to an imaging point at a predetermined distance from the principal point of the light-focusing optical section. In the present application, a beam diameter at the light focusing point is referred to as a focused beam diameter. Laser processing is performed by conforming the light focusing point to a portion to be processed of a workpiece, or by purposely displacing the light focusing point from the portion to be processed in a direction along the optical axis. The convergent angle affects the focusing depth of the laser beam at the portion to be processed, and the focused beam diameter affects the energy density of the laser beam at the portion to be processed. Thus, it is possible to determine optimum convergent angle and optimum focused beam diameter in accordance with the kind of laser processing, such as welding, cutting, marking, etc., or the material, thickness, etc. of the workpiece.
The quality of a laser beam used for laser processing depends on a focusing performance (M2) of the laser beam, and can be expressed by the product of a convergent angle and a focused beam diameter. The product of the convergent angle and the focused beam diameter is a constant value determined according to the kind of a laser beam (a laser medium, etc.), and it may be considered that the smaller the product the higher the laser beam quality. In the laser processing apparatus, when the laser beam entering the processing head has a minimum beam diameter and a spreading angle (also referred to as a diffusion angle, in the present application), the light-focusing optical section is generally designed so that the product of the convergent angle and the focused beam diameter of the output laser beam becomes equal to the product of the diffusion angle and the minimum beam diameter of the input laser beam (i.e., the light-focusing optical section does not affect the quality or focusing performance of the laser beam). In this connection, the focused beam diameter is determined by the minimum beam diameter of the input laser beam and the imaging magnification of the light-focusing optical section, and the convergent angle is determined by the diffusion angle of the input laser beam and the imaging magnification of the light-focusing optical section. As a result, in a case where it is desired, for example, to adjust the focused beam diameter due to a change in the kind of processing or the kind of workpiece, changing the imaging magnification of the light-focusing optical section causes a change in the convergent angle, and thus it is difficult to adjust only the focused beam diameter.
A laser processing apparatus configured to be able to adjust a focused beam diameter or a convergent angle, provided to the laser beam by a light-focusing optical section, has been known. For example, Japanese Unexamined Patent Publication No. 2012-024782 (JP2012-024782A) discloses a laser processing apparatus in which provisions are made so that a laser beam transmitted from a laser oscillator through a feeding fiber is either passed through a selected one of a plurality of process fibers of different core diameters or directed to a laser processing unit (or a light-focusing optical section) without passing through any process fiber. In this laser processing apparatus, it is claimed that the core diameter of the optical fiber immediately before the laser processing unit (i.e., the minimum beam diameter of the laser beam to be introduced into the laser processing unit) can be changed as needed, and therefore that the focused beam diameter of the laser beam to be focused by the laser processing unit can be adjusted according to the thickness, etc. of the workpiece without having to change the imaging magnification of the laser processing unit.
On the other hand, Japanese Unexamined Patent Publication No. 2009-056481 (JP2009-056481A) discloses a laser processing apparatus in which a laser beam emitted from a laser oscillator is transmitted through an optical fiber to an optical head (or a light-focusing optical section) and then a workpiece is irradiated with the laser beam, the apparatus being characterized in that an adjusting means for adjusting the spreading angle of the laser beam from the laser oscillator so as to become equal to or smaller than the allowable aperture of the optical fiber is provided just before the incident end of the optical fiber. In this laser processing apparatus, it is claimed that by adjusting the spreading angle of the laser beam (i.e., the collecting angle of the laser beam to be input to the optical fiber) on the input side of the optical fiber, any variation in the spreading angle of the laser beam emerging from the optical fiber (i.e., variations in the convergent angle and focused beam diameter of the laser beam focused through the optical head) can be eliminated and consistent processing results can always be obtained.
A laser processing apparatus configured to include means for adjusting the beam diameter or beam shape of the laser beam entering the light-focusing optical section, which is provided for purposes other than adjusting the convergent angle or focused beam diameter, has been also known. For example, Japanese Unexamined Patent Publication No. 2008-168333 (JP2008-168333A) discloses a laser soldering apparatus in which a circular laser beam entering a processing head from a laser oscillator is first converted into a ring-shaped laser beam by a beam converter constructed from a combination of a concave cone lens and a convex cone lens and then converged by a projection lens and projected in the shape of a ring onto the part to be processed on a workpiece. On the other hand, Japanese Unexamined Patent Publication No. 2009-178725 (JP2009-178725A) discloses a laser processing apparatus in which a laser beam from a laser source is shaped into an annular form by a laser shaping means including a pair of axicon lenses, and the annular laser beam is converged by a converging lens at a point for processing, thereby suppressing the spherical aberration of the converging lens.